1. Field of the Invention
The present invention relates to a data modulator/demodulator (MODEM) for a bidirectional data communication system which allows a plurality of computers, work stations or similar data terminals (DTEs) located at remote places in a building or on the premises to interchange data over metallic cables. More particularly, the present invention is concerned with a data MODEM of the type sending main data to be handled by the DTEs and subdata, e.g., network supervisory information to be handled by the data communication system to a metallic cable as a frequency division multiplexed transmit signal.
2. Description of the Prior Art
A data MODEM of the type described includes a modulator which converts the main data in the form of a unipolar signal to a main channel signal of baseband and converts the subdata in the form of a digital signal to a subchannel signal undergone FSK (Frequency Shift Keying) modulation and different in frequency from the main channel signal. The frequency division multiplexed transmit signal made up of the main channel and subchannel signals is fed out to a metallic cable, e.g., a parallel pair line or a coaxial cable. The main data may have any one of a plurality of bit rates (data rates) predetermined on a DTE basis. It is a common practice with the data MODEM to fix the output levels of the main channel and subchannel signals with no regard to the data rate.
The transmit signal fed out from the data MODEM to the metallic cable is sent to a demodulator included in a remote or opposite data MODEM without the intermediary of an exchange or a repeater. The demodulator demodulates the main channel signal and subchannel signal to reproduce the main data and subdata.
The attenuation of the transmit signal on the metallic cable increases with the increase in the transmission distance, and the attenuation constant of the cable is substantially proportional to the square root of frequency. Therefore, the attenuation of the transmit signal increases with the increase in the data rate or the carrier frequency of FSK modulation even when the cable length is constant. This brings about a problem that the transmittable distance of the transmit signal is limited by either of the main channel and subchannel signals since the output levels of the two signals have customarily been fixed, as stated earlier. As a result, the power available with the other channel signal cannot be sufficiently used. Specifically, assume that the carrier frequency for FSK modulation is constant, and that the maximum transmittable distance of the transmit signal is set when the main channel signal has a medium data rate. Then, the power of the main channel signal cannot be effectively used at lower data rates while the power of the subchannel signal cannot be effectively used at higher data rates. Moreover, when the output level of transmission is maintained constant to save the production cost and volume of the data MODEM, the transmittable distance of the main signal critically decreases with the increase in the data rate of the main data.